Three monkeys inoculated with the nonpathogenic molecular clone, SIVmac1A11, have been observed for up to 8 years for signs of simian AIDS. These animals have not developed clinical signs of disease and have maintained normal T lymphocyte counts. However, isolation of SIV from peripheral blood or lymph node has occurred at a very low, intermittent rate during this time. This indicates that the infection with an attenuated strain of SIVmac can be persistent, although at a very low level, and reversion to wild-type virulence has not occurred. The steady state level of viral replication, whether persistent or latent with intervals of reactivation, can be studied in these animals. Three monkeys that were originally inoculated with a partially attenuated molecular recombinant containing the genome of pathogenic SIVmac239, except for the envelope gp120 which was derived from attenuated SIVmac1A11, developed a moderate level persistent viremia and resistance to challenge with pathogenic SIVmac, as reported last year. These animals have remained clinically healthy, with normal T lymphocyte counts, up to 2 1/2 years after pathogenic virus challenge. SIV was not recovered from one of the three monkeys, from either PBMC or lymph node. The virus load in the other two monkeys remains very low, but has increased in lymph node up to 1 - 10 infected cells per million. These animals mimic the emerging small group of long-term survivors of HIV-1 infection. Antibody and cytotoxic T cell (CTL) activity in these animals continues to be monitored, and anti-SIV CTL activity has declined to undetectable levels. We are currently using these animals to assay another potential host immune response to SIV, delayed hypersensitivity. *KEY* molecular clones of simian immunodeficiency virus, Attenuation, Vaccine